Polyester film, laminate, and package

ABSTRACT

The present invention provides a polyester film suitable for use as a sealant, which not only has high heat sealing strength, but also is less liable to adsorb various organic compounds, and has a less shrinkage in heating and high tensile strength. Further, the present invention provides a laminate comprising at least one layer of the polyester film, and a packaging bag including the same. The polyester film which is formed with a polyester resin whose main constituent is ethylene terephthalate. The film has at least one heat sealing layer. The film has a predetermined heat sealing strength, a predetermined difference in reversing heat capacity measured with a temperature modulated DSC, and a predetermined heat shrinkage. Further, the laminate comprises at least one layer of the polyester film, and the packaging bag includes the same.

TECHNICAL FIELD

The present invention relates to a polyester film which is excellent in heat sealing strength, and a laminate and a packaging bag including the same.

BACKGROUND ART

Conventionally, sealant films are used as packaging materials for many of articles on the market represented by foods, pharmaceuticals and industrial products. The innermost layer of a packaging material constituting a packaging bag, a lid member or the like, is provided with a sealant layer made of a polyolefin resin such as polyethylene and polypropylene, an ionomer, or a copolymer resin such as EMMA which exhibits high sealing strength. These resins are known to be capable of achieving high adhesion strength through heat sealing. However, an undrawn sealant film made of a polyolefin resin as described in Patent Document 1 easily adsorbs a component made of organic compounds such as fat or oil and flavor material, and accordingly, a packaging material using a sealant film as the innermost layer, namely, a layer to be brought into contact with a content, has a disadvantage that the aroma or taste of the content is easily changed. In a case where a sealant layer made of a polyolefin resin is used as the innermost layer of a packaging bag for a chemical product, a pharmaceutical, a food, or the like, it is necessary to take such a measure to include a larger amount of active ingredient of a content beforehand, and thus, a sealant layer made of a polyolefin resin is not suitable for the use, in many cases.

On the other hand, a sealant film made of an acrylonitrile resin as described in Patent Document 2 has a feature of being less liable to adsorb an organic compound contained in a chemical product, a pharmaceutical, a food or the like. However, an acrylonitrile film sometimes fails to obtain good sealing strength. In view of such problems, Patent Document 3 discloses a polyester film for sealant use with non-adsorptivity against organic compounds. However, there has been a problem, for example, that when the film of Example 1 of Patent Document 3 is used as a sealant and left in a high-temperature environment such as a vehicle in mid-summer (about 80° C.), the sealant shrinks and an original shape cannot be maintained. To suppress the shrinking property of the film, there is also an attempt to use an undrawn polyester film as a sealant film. However, an undrawn polyester film has had a problem that, in a case of being subjected to a process such as heat sealing or printing, the film cannot withstand tensile force from a machine, for low tensile strength thereof, and is deformed or broken.

PATENT DOCUMENTS

Patent Document 1: JP-A-2002-256116

Patent Document 2: JP-A-H7-132946

Patent Document 3: WO02014/175313

NON-PATENT DOCUMENTS

Non-Patent Document 1: R. Androsch, B. Wunderlich, Polym., 46, 12556-12566 (2005)

Non-Patent Document 2: C. Lixon, N. Delpouve, A. Saiter, E. Dargent, Y. Grohens, Eur. Polym. J., 44, 3377-3384 (2008)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

An object of the present invention is to solve the problems of the conventional technique. That is, a gist of the present invention is to provide a polyester film suitable for use as a sealant, which not only has high heat sealing strength, but also is less liable to adsorb various organic compounds, and has a less shrinkage in heating and high tensile strength. The present invention also intends to provide a laminate comprising at least one layer of the polyester film suitable for use as a sealant, and a packaging bag including the same.

Means for Solving the Problems

The present invention has the following constitution.

1. A polyester film which is formed with a polyester resin whose main constituent is ethylene terephthalate, and satisfies the following requirements (1) to (5):

(1) the film has at least one heat sealing layer, and wherein the heat sealing layer presents on at least one of the films surfaces;

(2) the film has a pealing strength of 4 N/15 mm or more and 25 N/15 mm or less when the heat sealing layers of the polyester film are heat-sealed to each other at 160° C. and 0.2 MPa for 2 seconds;

(3) the heat sealing layer of the polyester film has a difference in reversing heat capacity of 0.18 J/g·K or more and 0.35 J/g·K or less, and wherein the difference in reversing heat capacity is measured between at a lower and a higher temperature than a glass transition temperature with a temperature modulated DSC;

(4) the film has a heat shrinkage of 0% or more and 15% or less both in a longitudinal direction and in a width direction when treated in hot water at 80° C. for 10 seconds; and

(5) the film has a tensile breaking strength of 80 MPa or more and 300 MPa or less either in a lengthwise direction or in a transverse direction.

2. The polyester film according to above 1, wherein a polyester component constituting the polyester film comprises at least one selected from the group consisting of neopentyl glycol, 1,4-cyclohexanedimethanol, isophthalic acid, and diethylene glycol.

3. The polyester film according to above 1 or 2, wherein a polyester component constituting the polyester film comprises 1,4-butanediol.

4. The polyester film according to any one of above 1 to 3, having a film thickness of 5 to 200 μm.

5. A packaging bag including at least one of the polyester film according to above 1 to 4.

6. A laminate comprising at least one layer of the polyester film according to above 1 to 4.

7. A packaging bag including at least one of the laminate according to above 6.

Effects of the Invention

The polyester film of the present invention not only exhibits high heat sealing strength, but also is less liable to adsorb various organic compounds, and therefore, is capable of hygienically packaging an article which contains oil or a flavor material, such as a chemical product, a pharmaceutical, and a food. In addition, the film has a small shrinkage when heated, and therefore, has a small shrinkage even in a high temperature environment. In addition, even when the polyester film is processed, problem such as breakage is less liable to occur. The present invention is further capable of providing a laminate comprising at least one layer of the polyester film, and a packaging bag including the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows reversing heat capacity curves of the heat sealing layer (film 1) of Example 1 and the heat sealing layer (film 3) of Comparative Example 1, as measured by a temperature modulated DSC.

MODE FOR CARRYING OUT THE INVENTION

A polyester film of the present invention is formed with a polyester resin whose main constituent is ethylene terephthalate, and satisfies the following requirements (1) to (5):

(1) the film has at least one heat sealing layer, and wherein the heat sealing layer presents on at least one of the films surfaces;

(2) the film has a pealing strength of 4 N/15 mm or more and 25 N/15 mm or less when the heat sealing layers of the polyester film are heat-sealed to each other at 160° C. and 0.2 MPa for 2 seconds;

(3) the heat sealing layer of the polyester film has a difference in reversing heat capacity of 0.18 J/g·K or more and 0.35 J/g·K or less, and wherein the difference in reversing heat capacity is measured between at a lower and a higher temperature than a glass transition temperature with a temperature modulated DSC;

(4) the film has a heat shrinkage of ·10% or more and 10% or less both in a longitudinal direction and in a width direction when treated in hot water at 80° C. for 10 seconds; and

(5) the film has a tensile breaking strength of 80 MPa or more and 300 MPa or less either in a lengthwise direction or in a transverse direction.

The polyester film of the present invention which satisfies the above requirements is a polyester film excellent in a heat sealing property, and suitable for use as a sealant. Since the film is less liable to adsorb various organic compounds, it is also possible to provide a sealant material suitable to a packaging bag. Further, since the film has a small shrinkage when heated, it is possible to maintain a shape of the film, even in a high temperature environment. In addition, the polyester film of the present invention has a good processability, for its high tensile strength.

In particular, since each of the heat sealing property and the low shrinkage; and each of the heat sealing property and the high tensile strength, are both properties antinomic to each other, there has conventionally been no polyester film which can satisfy all of these properties. Hereinbelow, the polyester film of the present invention is described.

-   1. Types of the polyester raw materials constituting the heat     sealing layer

The polyester used in the heat sealing layer of the present invention is a polyester whose main constituent component is ethylene terephthalate unit.

In addition, it is preferred that the polyester used in the present invention contains one or more monomer components which can be amorphous components (hereinbelow simply referred to as amorphous component). This is because presence of an amorphous component makes a difference in reversing heat capacity, which will be described later, less liable to lower, even during a film forming process such as drawing or heat setting, and heat sealing strength is improved. Examples of monomer of carboxylic acid component which can be an amorphous component may include isophthalic acid, 1,4-cyclohexanedicarboxylic acid, and 2,6-naphthalenedicarboxylic acid.

Further, examples of monomer of a diol component which can be an amorphous component may include neopentyl glycol, 1,4-cyclohexanedimethanol, diethylene glycol, 2,2-diethyl 1,3-propanediol, 2-n-butyl-2-ethyl-1,3-propanediol, 2,2-isopropyl-1,3-propanediol, 2,2-di-n-butyl-1,3-propanediol, and hexanediol.

Among these amorphous carboxylic acid components and diol components, it is preferred to include isophthalic acid, neopentyl glycol, 1,4-cyclohexanedimethanol, or diethylene glycol. Use of these components increases the difference in reversing heat capacity of the film, to facilitate improvement of heat sealing strength.

In the present invention, it is also possible to include a component other than ethylene terephthalate or amorphous components. Examples of dicarboxylic acid component constituting the polyester may include aromatic dicarboxylic acids such as orthophthalic acid; aliphatic dicarboxylic acids such as adipic acid, azelaic acid, sebacic acid and decanedicarboxylic acid; and alicyclic dicarboxylic acids. However, a polycarboxylic acid having three or more valences (for example, trimellitic acid, pyromellitic acid, and anhydrides thereof) is preferably not contained in the polyester.

In addition to those described above, examples of a component constituting the polyester may include long chain diols such as 1,4-butanediol, aliphatic diols such as hexanediol, and aromatic diols such as bisphenol A. Among them, 1,4-butanediol is preferably contained. Further, as a component constituting the polyester, a polyester elastomer containing ε-caprolactone, tetramethylene glycol, or the like, may be contained. These components have not only an effect of increasing the difference in reversing heat capacity of the film, but also an effect of lowering the melting point of the film, and are therefore preferred as a component of the heat sealing layer. However, it is preferred that the polyester do not contain a diol having 8 or more carbon atoms (e.g., octanediol, etc.), or a polyalcohol having 3 or more valences (e.g., trimethylolpropane, trimethylolethane, glycerin, diglycerin, etc.) which lowers film strength significantly.

The polyester film of the present invention may be added with various additives such a wax, an antioxidant, an antistatic agent, a crystal nucleating agent, a viscosity reducing agent, a thermal stabilizer, a coloring pigment, an anti-coloring agent, and an ultraviolet absorber. In addition, fine particles as a lubricant for improving smoothness of the film are preferably added to at least a surface layer of the film. As the fine particles, those of an arbitrary substance may be selected. Examples of inorganic fine particles may include those of silica, alumina, titanium dioxide, calcium carbonate, kaolin, barium sulfate, etc., and examples of organic fine particles may include those of acrylic resin particles, melamine resin particles, silicone resin particles, cross-linked polystyrene particles, etc. An average particle size of the fine particles may be appropriately selected according to needs within a range of 0.05 to 3.0 μm as measured by Coulter counter.

As a method of blending the particles in the polyester film of the present invention, for example, they can be added in an arbitrary step in production of the polyester resin, but they are preferably added in a step of esterification, or in a step before start of polycondensation reaction and after completion of ester exchange reaction as slurry dispersed in ethylene glycol etc., followed by carrying out polycondensation reaction. Further, the method may also include such as a method in which slurry of particles dispersed in ethylene glycol, water, other solvent or the like and raw materials of polyester resin are mixed using a kneading extruder with a vent, or a method in which dried particles and raw materials of polyester resin are mixed using a kneading extruder.

-   2. Layer structure of the polyester film

The polyester film of the present invention includes at least one layer of the heat sealing layer described above, and the heat sealing layer presents on at least one of the films surfaces. The polyester film of the present invention may be a single layer of a heat sealing layer alone, or may be a laminate structure of two or more layers. In a case where the film has a laminate structure, it is necessary that the heat sealing layer presents on at least one of the films surfaces. In laminating the film, it is possible to use a publicly-known method such as: a co-extrusion by a multi-manifold T die or an inflation method; adhesion by a wet or dry lamination or a hot melt; etc. From a viewpoint of strength, the heat sealing layer is preferably drawn at least in one direction (uniaxial drawing), and a biaxial drawing is more preferred, although any of undrawing, uniaxial drawing, and biaxial drawing may be used. A suitable manufacturing method in the case of biaxial drawing will he described later.

As a raw material used in a layer other than the heat sealing layer, it is possible to use the same polyester as one used in the heat sealing layer. For example, it is possible to use polyester which contains the aforementioned constituent components suitable to the heat sealing layer, providing a polyester layer different from the heat sealing layer in compositions. An amount of an amorphous component in a polyester raw material to be used in a layer other than the heat sealing layer is preferably 25 mol % or less. In a case where the amount of the amorphous component in a layer other than the heat sealing layer is more than 25 mol %, the film has to have low mechanical strength and a low heat resistance. From a viewpoint of strength, a layer other than the heat sealing layer is preferably drawn at least in one direction (uniaxial drawing), and a biaxial drawing is more preferred, although any of non-drawing, uniaxial drawing, and biaxial drawing may be used. A suitable manufacturing method in the case of biaxial drawing will be described later. It is further possible, in the polyester film of the present invention, to provide a layer processed by corona treatment, coating treatment, flame treatment or the like, in order to improve an adhesive property of a film surface irrespective of the heat sealing layer or other layers. Such a layer may be provided arbitrarily within a range not departing from requirements of the present invention.

-   3. Properties of the polyester film

Next, properties of the polyester film of the present invention, required in being used as a sealant, are described.

-   3.1. Heat sealing strength

First, it is preferred that a heat sealing strength of the polyester film of the present invention is 4 N/15 mm or more and 25 N/15 mm or less when the heat sealing layers are heat-sealed to each other at a temperature of 160° C., with a seal bar pressure of 0.2 MPa, for a sealing time of 2 seconds.

If the heat sealing strength is less than 4 N/15 mm, a sealed portion is easily peeled, and the film cannot be used as a packaging bag. The heat sealing strength is preferably 5 N/15 mm or more, more preferably 6 N115 nun or more. Although high heat sealing strength is preferred, a currently available upper limit is about 25 N/15 mm.

-   3.2. Difference in reversing heat capacity

It is preferred that the heat sealing layer of the polyester film has a difference in reversing heat capacity of 0.18 J/g·K or more and 0.35 J/g·K or less, and wherein the difference in reversing heat capacity is measured between at a lower and a higher temperature than a glass transition temperature (Tg) with a temperature modulated DSC.

Hereinbelow, a relationship between a concept of mobile amorphous phase and heat sealing strength is described.

Heat sealing is a technique which is worked out by softening or liquefying a heat sealing layer by heating, namely, by changing an arrangement of a polymer which is a constituent component. It is a common recognition that those cause a softening or liquefying easily with heat are amorphous components which has a weak binding of a molecular chain. Conventionally, it has been considered that a higher-order structure of a polymer constituting a film is divided into a crystalline phase and an amorphous phase, and that an amorphous phase is increased and heat sealing strength is enhanced simply by increasing an amount of an amorphous component. However, as a result of investigations by the present inventors, it has been found that, in a uniaxial drawing film or a biaxial drawing film, even if an amount of an amorphous component is simply increased, no increase in heat sealing strength corresponding to an increment amount is observed, and that the tendency is particularly remarkable in a biaxial drawing film. From these facts, it was conceived that a mobile amorphous content, which will be described below, was contributing to heat sealing strength.

It is known that an amorphous phase is further divide into rigid amorphous phase and mobile amorphous phase according to softness thereof (for example, Non-Patent Document 1). The mobile amorphous phase, which is the softest component among these three phases, is changed from a solid phase to a liquid phase with Tg as the boundary, in a process of temperature rise, to increase a heat capacity. On the other hand, in a rigid amorphous phase or a crystal phase, change from a solid phase does not occur until a melting point is reached, and accordingly, a difference in heat capacity between at a lower and a higher temperature than Tg corresponds to a mobile amorphous content. It is considered that, when a mobile amorphous content is large, a molecular chain become easy to move, namely, become easy to soften by heat, increasing an intrusion or fusing of the heat sealing layers to each other, resulting higher heat sealing strength. The inventors of the present invention have found that it is possible to secure desirable heat sealing strength by controlling a mobile amorphous content of a heat sealing layer to be within a prescribed range. If the difference in reversing heat capacity ΔCp is less than 0.18 J/g·K, a mobile amorphous content necessary to a heat sealing layer cannot be met, and heat sealing strength falls below 4 N/15 mm. On the other hand, the higher the difference in reversing heat capacity ΔCp of a heat sealing layer is, the more the heat sealing strength of the heat sealing layer is increased, which is preferred.

However, when the ΔCp of a heat sealing layer becomes too high, an appropriate heat sealing become difficult, since a heat resistance is lowered, and a blocking occurs at the time of heat sealing, in a periphery of a portion to be sealed (a phenomenon that a region broader than intended is sealed due to a heat conduction from a heating member). An upper limit of preferred difference in reversing heat capacity ΔCp is 0.4 J/g·K.

-   3.3. Shrinkage

A hot-water heat shrinkage in the polyester film of the present invention is 0% or more and 15% or less both in the width direction and the longitudinal direction, when treated in hot water of 80° C. for 10 seconds.

When the shrinkage exceeds 15%, a shrinkage becomes large when a film is heat-sealed, deteriorating flatness after the sealing. An upper limit of the hot-water heat shrinkage is preferably 14% or less, more preferably 13% or less. On the other hand, a hot-water heat shrinkage below zero means that a film is elongated, which is not preferred because it becomes hard to maintain an original shape of a film, similarly in the case where a shrinkage is high.

-   3.4. Film thickness

A thickness of the polyester film of the present invention is preferably 3 μm or more and 200 μm or less, although not particularly limited. A film thickness thinner than 3 μm is not preferred for causing a lack of heat sealing strength or making a process such as a printing difficult. A film thickness of thicker than 200 μm may also be possible, but is not much preferred because it increases weight of the film to be used, to increase a chemical cost. The film thickness is more preferably 5 μm or more and 160 μm or less, and even more preferably 7 μm or more and 120 μm or less.

-   3.5. Thickness irregularity in the longitudinal direction

A thickness irregularity of the polyester film of the present invention is preferably 18% or less when a measured length is 10 m in the longitudinal direction. A value exceeding 18% of the thickness irregularity in the longitudinal direction is not preferred, because it tends to cause a printing error when the film is subjected to a printing. Incidentally, the thickness irregularity in the longitudinal direction is more preferably 16% or less, and particularly preferably 14% or less. Although the smaller the thickness irregularity in the longitudinal direction is, the more it is preferred, the lower limit thereof is considered to be about 1% at the minimum, in view of performance of a film forming apparatus.

-   3.6. Thickness irregularity in the width direction

In the width direction, thickness irregularity is preferably 18% or less, when a measured length is 1 m. A value exceeding 18% of the thickness irregularity in the width direction is not preferred, because it tends to cause a printing error when the film is subjected to a printing. Incidentally, the thickness irregularity in the width direction is more preferably 16% or less, and particularly preferably 14% or less. Although the nearer to 0% the thickness irregularity in the width direction is, the more it is preferred, 1% is considered to be adequate for the lower limit, in view of performance of a film forming apparatus and easy producibility

-   3.7. Tensile breaking strength

A tensile breaking strength of the polyester film of the present invention is preferably 80 MPa or more and 300 MPa or less, at least in either the longitudinal direction or the width direction of the film. A tensile breaking strength below 80 MPa is not preferred, because it tends to cause a deformation or a breakage when a process such as a heat sealing or a printing is conducted. The tensile breaking strength is more preferably 90 MPa or more, and even more preferably 100 MPa or more. Although the higher the tensile breaking strength is, the tougher a film becomes, which is preferred, in the polyester film of the present invention it is difficult to exceed 300 MPa, and accordingly, un upper limit is 300 MPa.

-   4. Manufacturing method of the polyester film -   4.1. Film forming by co-extrusion -   4.1.1. Melt extrusion

The polyester film of the present invention is produced by using polyester as described in the above 1. “Types of the polyester raw materials constituting the heat sealing layer” as a raw material. In the present invention, it is possible to use, as a sealant layer, an undrawn film obtained by melt-extruding a polyester raw material. However in this case, in order to enhance strength of the film, it is preferred that, the film is laminated with a polyester film drawn in at least one direction by the method listed in the above 3. “Layer structure of the polyester film”. In the present invention, it is also possible to obtain the polyester film by subjecting an undrawn film obtained by melt-extruding a polyester raw material to a uniaxial drawing or a biaxial drawing according to a prescribed method described below. In a case of providing a layer in addition to a sealant layer at the time of melt extrusion, undrawn laminated films are obtained by conducting melt extrusion by separate extruder for individual layer. In this connection, it is possible to obtain polyester so as to contain an appropriate amount of monomers which can be amorphous components as described above, by selecting kind and amount of a dicarboxylic acid component and a diol component, which are then polycondensed. It is also possible that, two or more kinds of chip-shaped polyesters are mixed to be used as raw materials of the film. When a raw material is melt-extruded, it is preferable to dry the polyester raw material using a dryer such as a hopper dryer and a paddle dryer, or a vacuum dryer. After the polyester raw material is dried in such a manner, utilizing an extruder, it is melted at a temperature of 200 to 300° C., and extruded into an undrawn film. In such an extrusion, an arbitrary conventional method such as a T-die method and a tubular method can be adopted.

Then, the sheet-like molten resin after extrusion is quenched to be able to obtain an undrawn film. As a method for quenching the molten resin, a method in which a molten resin is cast on a rotary drum from a die and solidified by quenching to obtain a substantially unoriented resin sheet can be suitably adopted. The film is preferably drawn in at least one direction of lengthwise (longitudinal) direction and transverse (width) direction, that is, uniaxial drawing or biaxial drawing is preferred. Below described is a sequential-biaxial drawing method performed as lengthwise drawing-transverse drawing that first performs a lengthwise drawing and next performs a transverse drawing. The method may also be performed as transverse drawing—lengthwise drawing that reverses the order, since it merely changes the direction of main orientation. It is also possible to use a simultaneous biaxial drawing method.

-   4.1.2. Lengthwise drawing

The lengthwise drawing may preferably be performed by introducing an undrawn film to a lengthwise drawing machine in which a plural number of rolls are continuously disposed. In a lengthwise drawing, it is preferred that preheating be performed by a preheating roll, until a film temperature reaches 65° C. to 90° C. The film temperature lower than 65° C. makes a drawing in the lengthwise direction difficult, and accordingly, breakage tends to occur, which is not preferred. The temperature higher than 90° C. makes the film liable to stick to a roll, making the film liable to wind around a roll or making a roll liable to be contaminated from a successive production, which is not preferred.

A lengthwise drawing is conducted when the film temperature has reached 65° C. to 96° C. A lengthwise drawing ratio is preferably 1 time or more and 5 times or less. Since the 1 time means that lengthwise drawing has not been performed, the lengthwise drawing ratio should be 1 time in order to obtain a transverse uniaxial drawn film, and the lengthwise drawing should be 1.1 times or more in order to obtain a biaxial drawn film. Although an upper limit of the lengthwise drawing ratio may be any times, a too high lengthwise drawing ratio makes a transverse drawing difficult, making a breakage easily occur. Therefore, the upper limit is preferably 5 times or less. In addition, after the lengthwise drawing, a shrinkage in the longitudinal direction of the film, caused by the lengthwise drawing, may be reduced by relaxing the film in the longitudinal direction (relaxation in the longitudinal direction). It is also possible, by the relaxation in the longitudinal direction, to reduce a bowing phenomenon (distortion) which occurs in a tenter. Since the polyester film of the present invention contains an amorphous material, a shrinking property in the longitudinal direction caused by the lengthwise drawing is considered dominant to bowing distortion. That is because, in the subsequent steps of transverse drawing or the final heat treatment, heating is performed in such a state that both ends in the film width direction are held, and therefore, only the center portion of the film shrinks in the longitudinal direction. A relaxation ratio in the longitudinal direction is preferably 0% or more and 70% or less (relaxation ratio 0% means that relaxation is not conducted). Since an upper limit of the relaxation ratio in the longitudinal direction is determined depending on materials to be used or conditions in the lengthwise drawing, it is impossible to carry out relaxation beyond the upper limit. In the polyester film of the present invention, an upper limit of the relaxation ratio in the longitudinal direction is 70%. Relaxation in the longitudinal direction may be carried out by heating the film after the lengthwise drawing at a temperature of 65° C. to 100° C., and controlling the difference in rotation speed between the rolls. As a means of the heating, it is possible to use any of a roll, a near infrared ray, a far infrared ray, a hot air heater, and the like. The relaxation in the longitudinal direction is not necessarily carried out after the lengthwise drawing, but may be carried out at an arbitrary timing. For example, it is possible to carry out the relaxation during the transverse drawing (including a preheating zone) or during the final heat treatment, by narrowing a clip interval in the longitudinal direction (in this case, both ends in the film width direction are also relaxed in the longitudinal direction, and accordingly, the bowing distortion is reduced). After the relaxation in the longitudinal direction (in a case where the relaxation is not conducted, the lengthwise drawing), the film preferably be cooled once, and preferably be cooled on a cooling roll having a surface temperature of 20 to 40° C.

-   4.1.3. Transverse drawing

It is preferred that, after the lengthwise drawing, a transverse drawing is conducted with holding both ends in the width direction of the film by clips in a tenter, at 65° C. to 110° C., at a drawing ratio of 3.5 to 5 times. It is preferred that preheating has been conducted before the transverse drawing, and that the preheating has been conducted until a surface temperature of the film reaches 75° C. to 120° C.

It is preferred that, after the transverse drawing, the film is passed through an intermediate zone in which no aggressive heating operation is carried out. The subsequent final heat treatment zone has a higher temperature against the transverse drawing zone in the tenter. Therefore, if an intermediate zone is not provided, heat of the final heat treatment zone (hot air itself or radiant heat) should flow into the transverse drawing step. In such a case, a temperature of the transverse drawing zone is not stabilized, to cause not only a deterioration in the film thickness accuracy, but also a variation in a physical property such as heat sealing strength and a shrinkage. Thus, it is preferred that the film after the transverse drawing is passed through the intermediate zone to allow a prescribed time to elapse, and thereafter, the final heating treatment is conducted. In this intermediate zone, it is important to block an associated flow that accompanies a running of a film, or hot air from the transverse drawing zone or the final heat treatment zone, such that, when a strip-shaped paper piece is suspended in the intermediate zone in a state where a film is not passed through, the paper piece hangs down almost completely in the vertical direction. For a passage time of the intermediate zone, about 1 second to 5 seconds is sufficient. If the passage time is shorter than 1 second, the length of the intermediate zone becomes insufficient, resulting in an insufficient heat blocking effect. On the other hand, if the intermediate zone is too long, although the longer the more preferred, equipment should become large. Therefore, about 5 seconds is sufficient for the length.

-   4.1.1 Final heat treatment

After passed through the intermediate zone, the film is preferably subjected to a heat treatment at a temperature equal to or higher than the transverse drawing temperature and equal to or less than 180° C. in the final heat treatment zone. When the heat treatment temperature is not equal to or higher than the transverse drawing temperature, effect of the heat treatment is not exerted. In such a case, a shrinkage in hot water of 80° C. becomes higher than 15%, which is not preferred. Although the higher the heat treatment temperature becomes, the lower the shrinkage becomes, if the temperature becomes higher than 180° C., a haze of the film will exceed 15%, and the film will not be able to maintain transparency, which is not preferred. In the final heat treatment, it is possible to reduce a shrinkage in the width direction, by shortening a distance between the clips of the tenter at an arbitrary ratio (relaxation in the width direction). Thus, in the final heat treatment, it is preferred that relaxation in a range of 0% or more and 10% or less in the width direction be conducted. (relaxation ratio 0% means that relaxation is not conducted). Although the higher a relaxation ratio in the width direction is, the more a shrinkage in the width direction lowers, an upper limit of the relaxation ratio (a shrinkage in the width direction of a film immediately after a transverse drawing) is determined depending on raw materials used, drawing conditions in the width direction, or heat treatment temperature, and relaxation cannot be carried out beyond the upper limit. In the polyester film of the present invention, an upper limit of the relaxation ratio in the width direction is 10%. A time of passing through the final heat treatment zone is preferably 2 seconds or more and 20 seconds or less. If the passing time is 2 seconds or less, the film passes through the heat treatment zone without having a surface temperature reached a set temperature, which makes the heat treatment meaningless. The passing time is preferably 2 seconds or more, and more preferably 5 seconds or more, since the longer the passing time, the more the effect of the heat treatment is achieved. However, a longer passing time will result in an immense size of equipment, and 20 seconds or less is sufficient for a practical use. After that, the film is wound, with both ends cut and removed, to thus obtain a polyester film roll.

-   4.2. Adhesion between films

When the polyester film of the present invention is produced, it is also possible to adhere the polyester film mentioned in the above 4.1 “Film forming by co-extrusion” to another polyester film. When an undrawn film is used as the heat sealing layer, it is preferably adhered to a polyester film drawn in at least one direction. As a method of adhesion, the method listed in the above 3. “Layer structure of polyester film” may be used. In a case of dry lamination, it is possible to use a commercially available adhesive for dry lamination. Representative examples are Dick Dry (registered trademark) LX-703VL manufactured by DIC Corporation, KR-90 manufactured by DIC Corporation, TAKENATE (registered trademark) A-4 manufactured by Mitsui Chemicals, Inc., TAKELAC (registered trademark) A-905 manufactured by Mitsui Chemicals, Inc., etc.

EXAMPLES

Next, the present invention will be described more concretely by way of Examples and Comparative Examples. However, the present invention is by no means limited by modes of the Examples, and may appropriately be modified within a range not deviated from the gist of the present invention.

Evaluation methods of the film are as follows. Incidentally, in a case where a longitudinal direction and a width direction cannot immediately be identified for such a reason that an area of a film is small, the longitudinal direction and the width direction may provisionally be determined, since no problem is particularly caused even if the provisionally determined longitudinal direction and width direction are deviated by 90 degrees from correct directions.

<Evaluation Methods of Heat Sealing Layer> [Mobile Amorphous Content]

Samples of the heat sealing layers of 10.0±0.2 mg were weighed and put into a hermetic aluminum pan, and reversing heat capacity curves thereof were obtained by using a temperature modulated differential scanning calorimeter (DSC) “Q100” (manufactured by TA Instruments) in MDSC (registered trademark) heat-only mode, at an average temperature raising speed of 2.0° C./min., and with a modulation period of 60 seconds.

When a sealant layer is obtained from a laminated film, in a case of co-extruded film, a surface layer in a side of heat sealing layer was shaved with a feather blade. A cross section of the shaved film sample was observed with an electron scanning microscope (SEM) to check any layer other than heat sealing layer was not shaved. In a case of the laminate film, a notch was made in the film, from which the film was torn by hand, and interlayer exfoliation in the torn portion (cut) was peeled off with tweezers. For the peeled off heat sealing layer, a portion 1 cm or more away from the cut was sampled.

In the heat capacity curve obtained by the measurement, an inflection point was determined by using an attached analysis software (TA Analysis, manufactured by TA Instruments), and as a difference in heat capacity between at a lower and a higher temperature than the inflection point (a glass transition point), a difference in reversing heat capacity x ΔCp was determined according to the following equation 1.

Difference in reversing heat capacity ΔCp=(Heat capacity Cp1 in high temperature side)−(Heat capacity Cp2in low temperature side)   Equation 1

Here, in the heat capacity curve, an extension line of the base line of the heat capacity curve in the high temperature side relative to Tg was drawn, and an intersection point thereof with a tangent line of the inflection point (Tg) was determined. A value of Y axis corresponding to the intersection point (a reversing heat capacity) was read, and regarded as a heat capacity Cp1 in the high temperature side. An extension line of the base line of the heat capacity curve in the low temperature side relative to Tg was also drawn, and an intersection point thereof with a tangent line of the inflection point (Tg) was determined. A value of Y axis corresponding to the intersection point (a reversing heat capacity) was read, and regarded as a heat capacity Cp2 in the low temperature side.

<Evaluation Methods of Film> [Heat Sealing Strength]

Heat sealing strength was measured in conformity with JIS Z1707. Specific procedure is briefly shown. Heat sealing layers of samples which have not been subjected to a coating treatment or a corona treatment are adhered to each other with a heat sealer. Sealing conditions were an upper bar temperature set to 160° C., a lower bar temperature set to 100° C., a pressure was set to 0.2 MPa, and a time was set to 2 seconds. Adhesion samples were cut out so as to have a seal width of 15 mm. Peeling strength was measured by using a universal tensile strength tester “DSS-100” (manufactured by Shimadzu Corporation) at a tensile speed of 200 mm/minute. The peeling strength is shown in strength per 15 mm (N/15 min).

[Hot-Water Heat Shrinkage]

A film was cut into a square of 10 cm×10 cm, shrunk by being immersed in hot water of 80±0.5° C. for 10 seconds under a no load condition, then immersed in water of 25° C.±0.5° C. for 10 seconds, and taken out from the water. Thereafter, dimensions of the film in the lengthwise directions and transverse directions were measured, and a shrinkage in each directions was determined according to the following equation 2. Incidentally, the measurement was conducted twice, and average thereof was determined.

Shrinkage={(Length before shrinkage−Length after shrinkage)/Length before shrinkage}×100(%)   Equation 2

[Appearance After Left in High Temperature Environment]

A film was cut into a square of 10 cm×10 cm, and then laminated with another biaxial drawing polyester film E5100-12 μm (manufactured by Toyobo Co., Ltd.) similarly cut into a square of 10 cm×10 cm, by using an adhesive for dry lamination (TAKELAC (registered trademark) A-950 manufactured by Mitsui Chemicals, Inc.). This laminate was put in a thermo-hygrostat (IG400 manufactured by Yamato Scientific Co., Ltd.) in which temperature and humidity were set to 80° C./65% RH, and left for 24 hours. After 24 hours, the laminate was taken out, and the shrinkage determined from the above equation 2 was calculated. In a case that the shrinkage varies depending on a direction of the film, a shrinkage of a direction which had a larger shrinkage was taken. This shrinkage was evaluated as follows as an appearance after left in a high-temperature environment.

-   Judgment good shrinkage ratio relative to the original shape was     less than 2%. -   Judgment fair shrinkage ratio relative to the original shape was 2%     or more and 5% or less. -   Judgment poor shrinkage ratio relative to the original shape was     more than 5%.

[Thickness Irregularity in Longitudinal Direction]

A film was sampled in a shape of roll of 11 m in the longitudinal direction×40 mm in the width direction, and the thickness was measured continuously along the longitudinal direction of the film (measured length was 10 m), by using a continuous contact-type thickness meter manufactured by Micron Measuring Instrument Co., Ltd, at a measuring speed of 5 m/min. The thickness irregularity in the longitudinal direction of the film was calculated from the following equation 3, in which Tmax. is a maximum thickness, Tmin. is a minimum thickness, and Tave. is an average thickness at the time of measurement.

Thickness irregularity={(Tmax.−Tmin.)/Tave.}×100(%)    Equation 3

[Thickness Irregularity in Width Direction]

A film was sampled in a shape of wide belt of 40 mm length×1.2 m width, and thickness was measured continuously along the width direction of the film sample (measured length was 1 m), by using a continuous contact-type thickness meter manufactured by Micron Measuring Instrument Co., Ltd, at a measuring speed of 5 m/min. Thickness irregularity in the longitudinal direction of the film was calculated from the above equation 3, wherein Tmax. is a maximum thickness, Tmin. is a minimum thickness, and Tave. is an average thickness at the time of measurement.

[Tensile Breaking Strength]

A film sample in a rectangular shape of 140 mm in a direction to be measured and 20 min in a direction perpendicular to the measurement direction was prepared in conformity with JIS K7113. A tensile test was conducted by using a universal tensile tester “DSS-100” (manufactured by Shimadzu Corporation), with holding each 20 mm of both ends of a test piece with chucks (100 mm distance between chucks), under conditions of an ambient temperature of 23° C. and a tensile speed of 200 mm/min., and a stress at the time of tensile failure was taken as tensile breaking strength (MPa). Measurement directions were longitudinal direction and width direction, in which test was conducted individually.

[Aroma Retainability]

Two pieces of films were cut into squares of 10 cm×10 cm, which were then laminated with each other, and formed into a bag, with only one side open, and three sides heat-sealed at 160° C. Each 20 g of limonene (manufactured by NACALAI TESQUE, INC.) and menthol (manufactured by NACALAI TESQUE, INC.) were put therein, and then the opening one side was also heat sealed to produce a sealed bag. The bag was put in a glass container with a capacity of 1000 ml, and a lid was put on the glass container. After one week, the lid of the glass container was opened so that persons (total 16 persons with 4 persons in twenties, 4 persons in thirties, 4 persons in forties and 4 persons in fifties; the ratio of men to women was 1:1 in each age category) could smell the air in the glass container. The air in the glass container was smelled, and evaluations were made as follows.

-   Judgment good the number of persons who sensed a smell 0 to 1 -   Judgment fair the number of persons who sensed a smell 2 to 3 -   Judgment poor the number of persons who sensed a smell 4 to 16

[Adsorptivity]

A film was cut into a square of 10 cm×10 cm, and weighed. Then, the film was soaked into a solution of 500 ml in a container which was prepared by adding ethanol to limonene (manufactured by NACALAI TESQUE, INC.) and menthol (manufactured by NACALAI TESQUE, INC.) so that concentration of each became 30%, and taken out a week later. The film taken out was pressed with a Bemcot to remove the solution, and dried for 1 day in a room of a temperature of 23° C. and humidity of 60% RH. After dried, the film was weighed, and a difference in film weight determined from the following equation 3 was defined as an adsorbed amount.

Adsorbed amount=Film weight after immersion−Film weight before immersion   Equation 3

The adsorbed amount was judged as follows.

-   Judgment good 0 mg or more and 5 mg or less -   Judgment fair more than 5 mg and equal to or less than 10 mg -   Judgment poor more than 10 mg

<Preparation of Polyester Raw Material> [Synthesis Example 1]

A stainless steel autoclave equipped with a stirrer, a thermometer, and a partial reflux condenser was charged with 100 mol % of dimethyl terephthalate (DMT) as a dicarboxylic acid component, and 100 mol % of ethylene glycol (EG) as a polyalcohol component, such that, the ethylene glycol became 2.2 times of the dimethyl terephthalate in molar ratio, and then a transesterification reaction was conducted by using 0.05 mol % (relative to the acid component) of zinc acetate as a transesterification catalyst, with distilling off generated methanol from the system. Thereafter, 0.225 mol % (relative to the acid component) of antimony trioxide was added as a polycondensation catalyst, and a polycondensation reaction was conducted at 280° C., under a condition of reduced pressure of 26.7 Pa, to obtain polyester (A) with intrinsic viscosity of 0.75 dl/g. This polyester (A) is a polyethylene terephthalate.

[Synthesis Example 2]

Polyesters (B) to (F) were obtained in the same procedure as the Synthesis example 1, with changing the monomer. The compositions of each polyester are shown in Table 1. In Table 1, TPA is terephthalic acid, IPA is isophthalic acid, BD is 1,4-butanediol, NPG is neopentyl glycol, CHDM is 1,4-cyclohexanedimethanol, and DEG is diethylene glycol. In the production of polyester (E), SiO2 (Sylysia 266 manufactured by Fuji Silysia Ltd.) was added as a lubricant in a ratio of 7,000 ppm relative to the polyester. Each polyester was appropriately formed into a chip. The intrinsic viscosities of each polyester was B: 0.78 dl/g, C: 0.73 dl/g, D: 0.73 dl/g, E: 0.80 dl/g, F: 0.75 dl/g, respectively.

TABLE 1 composition of polyester raw material (mol %) dicarboxylic addition polyester acid amount of raw component diol component lubricant material TPA IPA EG BD NPG CHDM DEG (ppm) A 100 0 99 0 0 0 1 0 B 80 20 99 0 0 0 1 0 C 100 0 68 0 30 0 2 0 D 100 0 67 0 0 30 3 0 E 100 0 0 100 0 0 0 0 F 100 0 99 0 0 0 1 7000

The production method of each film is described below.

(Production of Polyester Film 1)

Polyester A, polyester C, polyester E, and polyester F were mixed together at a mass ratio of 5:66:24:5, as raw materials of a heat sealing layer (layer A). The mixed raw materials for layer A were fed into a biaxial screw extruder, melted at 270° C., discharged from a T die, and cooled on a chill roll with a surface temperature set to 30° C., to obtain an undrawn film formed only with layer A. The undrawn film obtained by cooling and solidification was introduced to a lengthwise drawing machine in which a plural number of rolls were arranged continuously, preheated on a preheating roll until the film temperature reached 80° C., and then drawn 4.1 times. The film immediately after the lengthwise drawing was passed through a heating furnace set by a hot air heater to 90° C., and subjected to a 20% relax treatment in the longitudinal direction, by utilizing the difference in rotation speed between the rolls at an entrance and an exit of the heating furnace. Thereafter, the lengthwise drawing film was forcibly cooled by a cooling roll with a surface temperature set to 25° C. The film after the relax treatment was introduced to a transverse drawing machine (tenter), subjected to preheating of 5 seconds until the surface temperature reached 95° C., and then drawn 4.0 times in the width direction (the transverse direction). The film after the transverse drawing was directly introduced to the intermediate zone, through which the film was passed through in 1.0 second. In this connection, in the intermediate zone of the tenter, hot air from the final heat treatment zone and hot air from the transverse drawing zone were blocked, such that, when a rectangular shaped piece of paper was dropped in the intermediate zone in a state where a film was not passed through, the piece of paper dropped in a substantially completely perpendicular direction. After that, the film passed through the intermediate zone was introduced to the final heat treatment zone, and subjected to a heat treatment at 125° C. for 5 seconds. At this time, a relaxation treatment of 3% in the width direction was conducted, by narrowing a clip interval in the film width direction, simultaneously with the heat treatment. After passed through the final heat treatment zone, the film was cooled, and wound into a roll shape with both ends cut and removed, so as to have a width of 500 mm, to produce a biaxial drawn film with a thickness of 30 μm continuously over a prescribed length. The production conditions are shown in Table 2.

(Production of Polyester Film 2)

Polyester A, polyester C, polyester E, and polyester F were mixed together at a mass ratio of 5:78:12:5, as raw materials of layer A. Polyester A, polyester C, polyester E, and polyester F were mixed together at a mass ratio of 5:54:36:5, as raw materials of the other layer (layer B). The raw material mixtures of layer A and layer B were each fed to separate biaxial screw extruders, and were melted at 270° C. Each molten resin was joined together by a feed block in the middle of a flow path, and discharged from a T die, and cooled on a chill roll with a surface temperature set to 30° C., to obtain an un drawn laminated film. The flow path for the molten resin was configured such that layer A formed both surface layers, and layer B formed the center layer of the laminated film (a 2 kind-3 layer structure of layer A/layer A/layer A), and an amount of discharge was controlled such that a thickness ratio of layer A and layer B became 50:50. Then, the film was formed into a biaxial drawn film with a width of 500 mm and a thickness of 30 μm, under the same conditions with polyester film 1 except that the final heat treatment temperature was set to 140° C. and the relaxation in the width direction was set to 15%. The production conditions are shown in Table 2.

(Production of Polyester Film 3)

An undrawn laminated film was obtained by using the same raw materials in the same mixing ratios with those of polyester film 2, for both layer A and layer B, which were melt-extruded in the same manner as in Example 2. Thereafter, the film was formed into a biaxial drawn film with a width of 500 mm and a thickness of 30 μm, under all the same conditions with polyester film 1, except that the final heat treatment temperature was set to 100° C. and the relaxation ratio in the width direction was set to 0%. The production conditions are shown in Table 2.

(Production of Polyester Film 4)

Polyester A, polyester C, polyester E, and polyester F were mixed together at a mass ratio of 10:75:10:5, as raw materials of layer A. Polyester A, polyester C, polyester E, and polyester F were mixed together at a mass ratio of 55:30:10:5, as raw materials of the other layer (layer B). These raw materials were each melt-extruded in the same manner as polyester film 2, to obtain am undrawn laminated film. Thereafter, the film was formed into a biaxial drawn film with a width of 500 mm and a thickness of 30 μm, under all the same conditions with Example 1, except that the lengthwise drawing temperature was set to 90° C., the transverse drawing temperature was set to 100° C., and the final heat treatment temperature was set to 115° C. The production conditions are shown in Table 2.

(Production of Polyester Film 5)

Polyester A, polyester C, polyester E, and polyester F were mixed together at a mass ratio of 32:53:10:5, as raw materials of layer A, and the mixture was melt-extruded in the same manner as polyester film 1 to obtain an undrawn film formed only with layer A. Thereafter, the film was formed into a biaxial drawn film with a width of 500 mm and a thickness of 30 μm, under the same conditions with polyester film 4. The production conditions are shown in Table 2.

(Production of Polyester Film 6)

Polyester A, polyester B, polyester D, polyester E, and polyester F were mixed together at a mass ratio of 5:86:2:2:5, as raw materials of layer A, and the mixture was melt-extruded in the same manner as polyester film 1, to obtain an undrawn film formed only with layer A. The film was wound into a roll shape with the both ends cut and removed, so as to have a width of 500 mm, to obtain an undrawn film having a thickness of 20 μm. The production conditions are shown in Table 2.

(Production of Polyester Film 7)

Polyester C and polyester F were mixed together at a mass ratio of 95:5 as raw materials of layer A, which were formed into a film under the same conditions with polyester film 6 in the subsequent steps, to obtain an undrawn film formed only with layer A. The production conditions are shown in Table 2.

(Production of Polyester Film 8)

Polyester A and polyester F were mixed together at a mass ratio of 95:5 as raw materials of layer B, and the mixture was melt-extruded by the same method with polyester film 1. Thereafter, a lengthwise drawing was conducted by the same method with polyester film 4, and the film was introduced to a tenter without subjected to a relaxation treatment in the longitudinal direction. Thereafter, the film was formed into a biaxial drawn film formed only with layer B, under the same conditions with polyester 1 except that the transverse drawing temperature was set to 110° C., and the final heat treatment temperature was set to 220° C. The production conditions are shown in Table 2.

TABLE 2 film 1 film 2 film 3 film 4 film 5 film 6 film 7 film 8 Raw material polyester A 5 5 5 10 32 5 0 — composition polyester B 0 0 0 0 0 86 0 — of heat polyester C 66 78 78 75 53 0 95 — sealing layer polyester D 0 0 0 0 0 2 0 — (A layer) polyester E 24 12 12 10 10 2 0 — (mass %) polyester F 5 5 5 5 5 5 5 — Raw material polyester A — 5 5 55 — — — 95 composition of polyester B — 0 0 0 — — — 0 layer (B layer) polyester C — 54 54 30 — — — 0 other than polyester D — 0 0 0 — — — 0 heat sealing polyester E — 36 36 10 — — — 0 layer (mass %) polyester F — 5 5 5 — — — 5 Layer structure A A/B/A AB/A A/B/A A A A B Thickness ratio of layer (layer A (%)/ 100/0 50/50 50/50 50/50 100/0 100/0 100/0 0/100 layer B (%)) Lengthwise drawing 80 80 80 90 90 — — 90 drawing temperature (° C.) drawing ratio 4.1 4.1 4.1 4.1 4.1 — — 4.1 Relaxation heating furnace 90 90 90 90 90 — — — treatment in the temperature (° C.) longitudinal relaxation ratio (%) 20 20 20 20 20 — — — direction Transverse drawing 95 95 95 100 100 — — 110 drawing temperature (° C.) drawing ratio 4.0 4.0 4.0 4.0 4.0 — — 4.0 Final heat temperature (° C.) 125 140 100 115 115 — — 220 treatment relaxation ratio in 3 15 0 3 3 — — 3 the width direction (%) Film thickness (μm) 30 30 30 30 30 20 20 10

[Evaluation of Film] EXAMPLE 1

In Example 1, an evaluation film was produced by laminating polyester film 1 as the heat sealing layer, and polyester film 8 as the other layer. The two films were adhered to each other by using an adhesive for dry lamination (TAKELAC (registered trademark) A-950 manufactured by Mitsui Chemicals, Inc.). Evaluation results of the obtained laminated film are shown in Table 3.

EXAMPLES 2 to 4

In Examples 2 to 4, the heat sealing layer and the other layer were also laminated with each other by the same method with Example 5. Polyester films 3, 6, and 7 were used as heat sealing layers of Examples 2, 3, and 4, respectively, and polyester film 8 was used as the other layer of each. Evaluation results of the obtained laminated films are shown in Table 3.

EXAMPLES 5 to 8

In Examples 5, 6, and 8, polyester films 1, 2, 4, and 5 were respectively used as a single layer. Evaluation results of each film are shown in Table 3.

COMPARATIVE EXAMPLES 1 to 3

In Comparative Examples 1, 2, and 3, polyester films 3, 6, and 8 were respectively used as a single layer. Evaluation results of each film are shown in. Table 3.

TABLE 3 Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Comparative Comparative Comparative ple 1 ple 2 ple 3 ple 4 ple 5 ple 6 ple 7 ple 8 Example 1 Example 2 Example 3 Layer heat sealing film 1 film 3 film 6 film 7 film 1 film 2 film 4 film 5 film 3 film 6 film 8 structure layer other layer film 8 film 8 film 8 film 8 — — — — — — — Total thickness (μm) 40 40 30 30 30 30 30 30 30 20 10 Difference in reversing heat 0.26 0.22 0.29 0.34 0.26 0.24 0.22 0.18 0.22 0.29 0.13 capacity of heat sealing layer ΔC_(p) (J/g · K) Heat sealing strength at 160° 15.1 14.3 18.1 22.8 14.6 13.1 12.2 5.1 Unmea- 18.6 0 C. (N/15 mm) surable Hot-water Lengthwise 1.8 0.8 0.3 0.7 5.5 1.8 4.8 4.8 9.4 0.5 0.1 shrinkage (%) direction 80° C. Transverse 4.4 1.2 0.1 0.2 13.5 −1.5 6.8 6.6 23.1 0.4 0.2 direction Thickness Lengthwise 14.3 13.7 10.5 9.4 13.5 12.4 3.2 3.0 13.1 8.4 2.5 irregularity (%) direction Transverse 16.8 14.9 11.3 8.8 15.6 14.6 5.4 5.5 15.3 7.4 4.7 direction Tensile Lengthwise 167 159 97 102 139 142 220 224 122 63 247 breaking direction strength (MPa) Transverse 185 188 103 104 169 164 256 258 149 61 245 direction Appearance after left in high good good good good fair fair fair fair poor good good temperature environment Flavor menthol good good good good good good good good good good good retainability limonene good good good good good good good good good good good Adsorptivity menthol good good good good good good good good good good good limonene good good good good good good good good good good good

[Evaluation Results of Films]

According to Table 3, good evaluation results were obtained in any of the films of Examples 1 to 4, with a difference in reversing heat capacity ΔCp of the heat sealing layer in a prescribed range, and excellent heat sealing strength, shrinkage, thickness irregularity; tensile breaking strength, appearance after left in a high-temperature environment, aroma retainability, and adsorptivity. Incidentally, the chart in FIG. 1 shows no disturbance, and the base line shifts in the vicinity of Tg, which confirms that the DSC measurement was performed properly. As for the films of Examples 5 to 8, although the appearance after left in a high-temperature environment was somewhat inferior to those of Examples 1 to 4, the heat sealing strength were measurable, and the films were found to have no problem in use. Any of the films of Examples 5 to 8 exhibited a difference in reversing heat capacity ΔCp of the heat sealing layer in a prescribed range, and was excellent in heat sealing strength, shrinkage, thickness irregularity, tensile breaking strength, aroma retainability, and adsorptivity. On the other hand, as for the film of Comparative Example 1, although the ΔCp of the heat sealing layer was satisfied, and thickness irregularity, tensile breaking strength, aroma retainability, and an adsorptive property were excellent, it was impossible to measure heat sealing strength, for having poor flatness after the heat sealing, due to a high shrinkage. Moreover, evaluation on appearance after left in high temperature environment was also low. As for the film of Comparative Example 2, although the ΔCp of the heat sealing layer was also satisfied, and heat sealing strength, shrinkage, thickness irregularity, aroma retainability, and the adsorptivity were also excellent, a requirement of the present invention was not satisfied for having low tensile breaking strength. The film of Comparative Example 3 marked heat sealing strength of zero, for having been peeled off immediately after heat sealed, due to a low ΔCp of the heat sealing layer.

INDUSTRIAL APPLICABILITY

The present invention relates to a polyester film excellent in heat sealing strength, which is capable of being suitably used in sealant use, not only for being excellent in heat sealing strength, but also for being less liable to adsorb various organic compounds, and for having a small shrinkage in heating, and high strength. At least one layer including the polyester film of the present invention is also capable of being laminated with another film, and it is also possible to provide packaging bag including such a laminate. 

1. A polyester film which is formed with a polyester resin whose main constituent is ethylene terephthalate, and satisfies the following requirements (1) to (5): (1) the film has at least one heat sealing layer, and wherein the heat sealing layer is present on at least one of the films surfaces; (2) the film has a peeling strength of 4 N/15 mm or more and 25 N/15 mm or less when the heat sealing layers of the polyester film are heat-sealed to each other at 160° C. and 0.2 MPa for 2 seconds; (3) the heat sealing layer of the polyester film has a difference in reversing heat capacity of 0.18 J/g·K or more and 0.35 J/g·K or less, and wherein the difference in reversing heat capacity is measured between at a lower and a higher temperature than a glass transition temperature with a temperature modulated DSC; (4) the film has a heat shrinkage of 0% or more and 15% or less both in a longitudinal direction and in a width direction when treated in hot water at 80° C. for 10 seconds; and (5) the film has a tensile breaking strength of 80 MPa or more and 300 MPa or less either in a lengthwise direction or in a transverse direction.
 2. The polyester film according to claim 1, wherein a polyester component constituting the polyester film comprises at least one selected from the group consisting of neopentyl glycol, 1,4-cyclohexanedimethanol, isophthalic acid, and diethylene glycol.
 3. The polyester film according to claim 1, wherein a polyester component constituting the polyester film comprises 1,4-butanediol.
 4. The polyester film according to claim 1, having a film thickness of 5 to 200 μm.
 5. A packaging bag including at least one of the polyester film according to claim
 1. 6. A laminate comprising at least one layer of the polyester film according to claim
 1. 7. A packaging bag including at least one of the laminate according to claim
 6. 8. The polyester film according to claim 2, wherein a polyester component constituting the polyester film comprises 1,4-butanediol.
 9. The polyester film according to claim 8, having a film thickness of 5 to 200 μm.
 10. A packaging bag including at least one of the polyester film according to claim
 9. 11. A laminate comprising at least one layer of the polyester film according to claim
 9. 12. A packaging bag including at least one of the laminate according to claim
 11. 